If you watch the video, it shows a Denver tackler flying into the scene and kneeing Carr in the back (somewhere near the kidney area). Immediately after the knee impact, Carr reaches around with his hand and grabs the same exact spot on his back where the knee impacted and twinges in pain.
So if kneeing a person in the back causes spasms, I guess I could believe it is spasms. Just seems like they would have said it is a contusion instead of spasms. Del Rio said "something caused his back to spasm up" I am guess that Del Rio said that not realizing he got kneed in the back. And that also might be why Carr responded with the comment "I'm glad he knows. That's awesome." Would make sense why he made the smart ass remark.